The present invention relates to a chuck drive system, especially for a rotating clamping device of a machine tool, and is directed in particular to a chuck drive system of a type including a rotating chuck cylinder receiving a piston subdividing the cylinder in two pressure chambers and having a piston rod rotatably supported in a stationary headstock, with the pressure chambers being supplied with a fluid under pressure via passageways through the piston rod, and with check valves being positioned in the fluid passageways for sealing the pressure chambers.
In general, a chuck is attached to the drive spindle of a machine tool and used to grip and actively engage a revolving workpiece, cutting tool, drill, tool holder etc., via a collet, whereby a displacement of the chuck in an axial direction between an advanced position and a withdrawn position will cause the collet to release or clamp the workpiece, cutting tool or the like. The pull-in and push-out forces are transmitted by the chuck most conveniently through supply of compressed oil to the pressure chambers by a stationary hydraulic mechanism by which the velocity of the piston and the pressure acting upon the piston is determined. In order to maintain and control these forces during rotation of the chuck, hydraulic fluid is continuously supplied to the opposing pressure chambers of the rotating cylinder via the headstock which runs with contactless gap seals. The clamping pressure is maintained through continuous supply of pressure fluid, e.g. compressed oil, during operation of the drive spindle, whereby leakage oil escaping from the sealing gaps represents a source of loss. A further source of loss is the viscous friction of oil within the sealing gaps which friction increases by the square of the rotational speed. Both these energy losses limit the application range of the conventional headstock and consequently the range of attainable rotational speeds and clamping forces.
German publication no. DE-OS 24 19 808 and European Pat. No. 0 220 134 B1 disclose chuck drive systems in which each of the pressure chambers can be hermetically sealed off by mutually unblocking check valves so that the supply of pressure fluid can be effectively shut off during operation of the cylinder. For safety reasons, the pressure inside the sealed-off pressure chambers must be continuously monitored. In order to minimize energy losses in the headstock, it is further necessary to ensure that pressure fluid is substantially purged from the sealing gaps during operation of e.g. the drive spindle, once the supply of pressure fluid is cut.
Conventionally, the clamping pressure inside the pressure chamber is monitored by mechanically operating devices in form of spring-biased floating secondary pistons positioned in the pressure chambers and securely joined together via a linkage for axially movement in unison with a ring which functions as a control disk. A fixed section of the drive supports a position sensor for axial displacement. Upon drop of the clamping pressure, the ring-shaped control disk is displaced which in turn is sensed by the position sensor which can be adjusted to the clamping pressure being monitored through axial displacement. Such a pressure control is not fully automatic since the operator is required to reset the position sensor by hand after the clamping pressure is adjusted in dependence on the workpiece. Consequently, the actions of the operator introduce a significant safety risk.
A further drawback of conventional chuck drive systems is their inability to differentiate between the prevalent pressures in the pressure chambers. In case a desired clamping pressure is effected in one of the pressure chambers while a residual pressure is still present in the other pressure chamber, only the difference in the two pressures is applied as a force to the piston rod, which means that only this resulting force is crucial for the force applied upon the piston rod. A residual pressure in the pressure chamber may be caused e.g. by a mechanical defect of the check valve which seals this pressure chamber and then fails to open. Conventional pressure monitoring systems are, however, unable to distinguish as to whether the desired clamping pressure is effected only in one pressure chamber or whether a residual pressure is still present in the other pressure chamber. Theoretically, a situation may be encountered in which the residual pressure reaches the desired clamping pressure so that the force applied to the chuck is effectively zero. Conventional pressure monitoring systems cannot recognize this extremely precarious operating state of the chuck cylinder but would simply indicate that the desired clamping pressure is applied. This is one of the reasons why such chuck cylinders have not yet been commercially implemented.
Also, since in conventional pressure monitoring systems only one position sensor is provided for both pressure chambers, this sensor will, of course, not recognize which of the pressure chambers experiences a build up of pressure and in which direction the piston rod will travel as a result. For the control of the machine tool, it is, however, important to receive confirmation that the piston rod travels in the desired direction and to know whether the piston rod applies a pull or a push force for actuating, e.g. a chuck.
In order to minimize energy losses during operation, German publication no. DE-OS 24 19 808 proposes to purge the oil from the gap seals in the headstock with compressed air. This requires however an additional energy source as well as separate valves, and separate annular grooves must be formed within the headstock. The air-oil mist which is created when the gap seal is purged, may in fact be impossible to control, since labyrinth seals as commonly used in these headstocks are ineffective when stationary.
European Pat. No. 0 220 124 B1 also describes a proposal to purge compressed oil from the gap seals by incorporating between the headstock (headstock frame) and the revolving part (spindle body) of the chuck cylinder a transmission block which is pressed against the chuck cylinder in axial direction only when hydraulic fluid is being introduced into one of the two pressure chambers. After the pressure chamber is filled, the transmission block is returned to its initial position by springs. The incorporation of the transmission block significantly complicates the hydraulic system which already is substantial as a result of the monitoring system. Moreover, the overall structural dimensions are increased and are hardly practicable to meet demanded space limitations. In addition, it would be difficult to determine the magnitude of the required contact force for the transmission block. While an excessive force results in an inadmissible metallic contact between the transmission block and the wall surface of the chuck cylinder, a force that is too small leads to excessive energy losses during transmission of pressure. In this way, the functionality of the system can become uncontrollable.